The ultimate object of the present invention is to produce or make possible the production of a curved glass member having a transparent, electroconductive coating. The invention is particularly adapted to producing such a member for use in heating applications at low voltages relative to prior art. Such glass might be used for CRT screens which you can input by touching, architectural glass (such coatings are infrared reflective), curved, heated mirrors, and heated windows.
One possible application for such a product is automotive windshields. By applying a current to such windshields, accumulated frost, ice or fog can be removed by heating. It has been found that one should be able to generate 0.6 watts per square inch within a windshield of typical shape, with a space of 25" between the electrical bus bars, in order to clear rime ice 0.1" thick in about five minutes. Using a 60 volt auxiliary power source, which systems are readily available for use in automobiles, this means that the sheet resistivity of any electroconductive coating on the glass must only be about 10 ohms per square.
Further, federal light transmissibility standard (FMVSS No. 205) requires a 70% minimum visible light transmission at an angle of 90 degrees to the glass surface. Further, the electroconductive coating used, in order to be suitable in appearance, must be very thin, uniform, low in absorption, and low in reflection. These constraints make it particularly difficult to achieve such low resistivity. Assuming a coating with a range of index of refraction of 1.6 to 2.1, as is the case with indium-tin oxide coatings, a nominal full wave coating would have a physical thickness of approximately 3,400 to 2,600 angstroms (i.e., the median wavelength for visible light, 5,500 A, divided by the index of refraction).
Prior to my work, there were no known commercially viable techniques available for providing satisfactory curved glass uniformly coated with such a thin film having such a low resistivity. Of course it is known to put transparent conductive coatings on aircraft windows, display cases and the like. Generally, however, flat glass is uscd in such applications. Also, high voltage power sources are available in such environments so that a high resistance in the film coating can be tolerated.
A common technique for preparing aircraft windows with electrically conductive coatings is to coat the hot surface of the glass with a tin oxide deposited pyrolytically. U.S. Pat. No. 2,954,454 discloses such a method for creating a coated, bent glass windshield. The problem with such a system is that in order to achieve coatings which make it possible to deliver 0.6 watts per square inch at low voltages, i.e., about 60 volts, one must provide a relatively thick film of between 5,000 and 10,000 angstroms. This results in a windshield or curved glass article which yields a "rainbow" of reflected color when exposed to light. This is partially due to the thickness of the coating, partially to the high index of refraction of tin oxide, and partially to the inherent nonuniform thickness of pyrolytically deposited tin oxide. Also it is suspected that such a process will yield a coating which will craze when the glass is bent, at least if produced on an economical commercial basis and using conventional bending procedures.
Indium-tin oxide has heretofore been sputter coated onto flat glass for use in liquid crystal display electrodes. However, when processed in accordance with conventional procedures, such indium-tin oxide, sputter coated flat glass cannot be bent on a viable production basis without crazing the indium-tin oxide coating. Such crazing of course interrupts the conduction of electricity and leads to serious imperfections in the heating pattern in the product.
United Kingdom Pat. No. 1,446,849, published Aug. 18, 1976, discloses the sputter coating of an already curved sheet of glass. It is believed that such a process would be slow, and uniformity is believed to be a serious problem. It is not believed that such a process would be acceptable in commercial production.
As a result of such drawbacks, it is believed that prior artisans have not heretofore commercially produced electrically conductive curved parts for low voltage by glass coating processes, though a concerted effort is now being made to introduce a heated windshield coated curved with zinc oxide and silver layers. Further, even alternatives other than coating the glass, such as embedded wires or embedded conductively coated plastic films, have not proven commercially acceptable for heating applications where visibility is primary.
I have heretofore found that a key to making an indium-tin oxide coated substrate bendable is to provide for a degree of oxygen substoichiometry in the oxide coating at the time of bend. The extent of substoichiometry must be sufficient to avoid crazing during bending, but not so great as to cause the final product to have less than 70% light transmission or be hazy. This method is disclosed and claimed in my U.S. Pat. No. 4,490,227.
In the foregoing patent, I disclose two specific ways to achieve a degree of substoichiometry at the moment of bend. The first involves initially coating the substrate to a low initial light transmittance (T.sub.o), i.e., between about 10 and about 40%. This is sufficiently low to insure a degree of substoichiometry even though the film is subsequently fired at atmosphere. The bending is also conducted in atmosphere.
The second method disclosed involves coating the substrate normally, i.e., to an initial light transmittance which may be as high as about 75%, followed by bending the coated glass in a reducing environment.
Both of these techniques for achieving substoichiometry at the moment of bend are acceptable commercial procedures. However, experience in practicing these previously disclosed techniques has taught that a degree of care has to be taken to avoid faster bends, i.e., bends at higher temperatures, deeper bends and thicker indium-tin oxide films.